When properly designed and constructed, rechargeable lithium-ion cells can exhibit excellent charge-discharge cycle life, little or no memory effect, and high specific and volumetric energy. However, lithium-ion cells do have some shortcomings, including an inability to tolerate recharging to potentials above the manufacturer's recommended end of charge potential without degradation in cycle life; the danger of overheating, fire or explosion for cells recharged to potentials above the recommended end of charge potential; and difficulties in making large cells having sufficient tolerance to electrical and mechanical abuse for consumer applications. Single and connected (e.g., series-connected) lithium-ion cells typically incorporate charge control electronics to prevent individual cells from exceeding the recommended end of charge potential. This circuitry adds cost and complexity and has discouraged the use of lithium ion cells and batteries in low-cost mass market electrical and electronic devices such as flashlights, radios, CD players and the like. Instead, these low-cost devices typically are powered by non-rechargeable batteries such as alkaline cells.
Various chemical moieties have been proposed for imparting overcharge protection to rechargeable lithium-ion cells. Chemical moieties designated as “redox shuttles” or “shuttles” may in theory provide an oxidizable and reducible charge-transporting species that may repeatedly transport charge between the negative and positive electrodes once the charging potential reaches a desired value. Materials that function as fuses or shunts to provide one-time or limited-time cell overcharge protection have also been proposed.
Phenothiazine compounds have been used for various purposes in lithium-ion cells. References involving such uses include U.S. Pat. No. 4,869,977 (Connolly et al.) and U.S. Pat. No. 5,976,731 (Negoro et al.), and Japanese Published Patent Application Nos. 5-295058 (Yuasa Corp.), 2001-15156 (Sony Corp. '156) and 2001-23687 (Sony Corp. '687).